Current methods of lamination for bonding, for instance, a clear optical substrate, such as glass or clear plastic, to another substrate can be achieved using acrylic tape, acrylic based film type adhesive material or acrylic based adhesive.
Substrate to substrate lamination methods for touch panel devices using acrylic tape as the securing means may introduce air spaces in between the substrates. The air spaces undesirably reduce optical clarity as the refractive index between air, and glass, polymethyl methacrylate (PMMA) or polyethylene terephthalate (PET) (typical optical lens material) are very different. FIG. 16 illustrates a prior art embodiment using acrylic tape 134 to adhere an optical lens substrate 132 and a touch panel substrate 136, and to adhere the touch panel substrate 136 to a Liquid Crystal Display (LCD) substrate 138. Air spaces 130 are disposed in between the substrates.
Lamination methods using film type adhesive material tend to have difficulties eliminating bubbles formed during lamination. The problem is especially tricky for large size substrates of 7 inches (diagonal measurement) and above. Film type adhesive material requires an additional step of die cutting the film material to specific sizes for usage. Furthermore, reworking a product laminated using film type adhesive material can be particularly difficult.
A conventional lamination method using film type adhesive material requires careful selection of several process parameters. Such process parameters include a roller (used for applying pressure over the film type adhesive material) tilt angle, speed of rolling, force of rolling, thickness of the film type adhesive, temperature of lamination, and so on. For big panel substrates (e.g. 7 inches diagonal and above), production yield is usually less than satisfactory despite careful selection of the process parameters.
Moreover, in the conventional lamination method, using film type adhesive material, laminated products or assemblies with bubbles undesirably formed during lamination need to be placed into an autoclave chamber for about 2 to 5 hours depending on the size of bubbles and the number of bubbles to reduce the bubbles to sizes not discernible to human eyes. Defective products with different degrees of bubble problems need to be separated into different batches, each batch having different exposure time in the autoclave chamber. In a mass production environment, separation of the defective products into different batches is difficult to manage and time consuming.
Display panel substrates formed using liquid-type Ultra Violet (UV) curable adhesives have significant disadvantages because existing techniques using these adhesives are in their infancy. For instance, several problems arise from use of existing liquid-type UV curable adhesives for the formation of display panel substrate assemblies. One particularly prevalent problem arises from the unwanted presence of air bubbles in the adhesive, leading to poor optical clarity of the substrate.
In addition, an ‘overflow’ problem exists for low viscosity adhesives currently in use. These adhesives are typically provided with a viscosity value in the range of between 1000 mpas to 15000 mpas. At such viscosity values, liquid resistance to flow is weak and the adhesive has tendency to overflow, i.e. spread or spill over undesirably. Efforts have been made to address this problem by seeking to dispense very carefully in a precise manner the right amount of adhesive to apply to a substrate area. However, this presents, indeed introduces, a fresh set of challenges in order to ensure that the amount of adhesive which is dispensed is of the right shape and size for a specific lamination process. Typically, the adhesive cannot be controlled to achieve the acceptable overflow of the adhesive at all the edges because the precision of adhesive dispensing volume cannot achieve the acceptable accuracy of less than 3% error rate. As a result, there may be numerous product rejects and yield is poor. Although reworking of defective products is possible for products laminated using adhesive, the process is repetitive and labor-intensive, thereby leading to delays in the manufacturing process, additional costs and/or less than desirable yield rates.
In an attempt to improve yield, some manufacturers use time based methods to provide time for the adhesive to flow to the laminated product or assembly edges before curing the adhesive using UV radiation. However, this does not solve the problem of lack of flow control due to low liquid viscosity and, indeed, lengthens the lamination process even further. The dispensed adhesive volume still cannot be controlled within the desired overflow tolerance. The adhesive flow out tolerances are typically 0.5 mm to 1 mm depending on product design requirements.
Overflow is undesirable as the overflowed adhesive could contaminate the surroundings of the substrate. Also, once overflowed adhesive has hardened after curing, the hardened adhesive could alter the dimensions of the laminated product, making the laminated product difficult for assembling into another part of the complete product, such as an electronic device.